In prebake aluminum smelters, carbon anodes are baked to elevated temperatures before they are delivered to the potrooms. The baking process takes place in refractory constructed ring furnaces where anodes are stacked in pits and surrounded with coke to prevent anode deformation and exposure to air during baking. The pits are bordered on each side by flues in which fuel is burned to provide heat. These anode pits are positioned in a matrix, typically either six or seven abreast, alternately sandwiched between the flues. These sections are arranged in line with adjoining sections so the flues are connected in series, in effect forming a continuous ring
Baking is performed by movement of independent fire groups over the baking sections, with packing of green anodes and removal of baked anodes occurring on either side of the sections undergoing bake. Each fire group consists of several burner bridges, a forced air cooling manifold, and an exhaust gas manifold. A burner bridge is comprised of a row of burners manifolded in parallel which are inserted into the flues of a section and individually regulated to achieve the desired flue temperature. The mobile forced draft air manifold located upstream of the burner bridges provides both the cooling air to the completed bake sections and, using this heat exchange, preheated combustion air to the fired sections. A mobile exhaust gas manifold draws the combustion gases through the fire group and directs them to an external fume treatment system. Once the desired final anode temperatures are reached in the final baking section, the entire fire group, including burner frames, cooling manifold and exhaust manifold, is repositioned one section downstream by overhead crane and another cycle is started. Typically, a fire group is moved every 24 hours, and a section of anodes completes the total cycle in about 20 days.
The firing equipment operates above the furnace and interfaces with the furnace through holes in each flue top. It is through these holes that the combustion air is induced, the fuel in introduced, and the spent gasses are extracted. Any instrumentation used to measure the conditions inside the flues is also inserted into these openings. Furnaces of this type have two to four smaller flue top openings of 3 to 5 inch diameter, and a larger opening of approximately 12 inch diameter (or square). The smaller holes are commonly called "peepholes," and are used for fuel input and instrumentation. These are always located on the flue top. The larger holes are known as exhaust port openings and are used for input of the cooling and combustion air at one end of the fire group, and for exhaust of spent gas at the other end. Depending on the furnace design, these holes are arranged one of two ways: one exhaust port opening on the headwall plus three or four peepholes on the flue top; and no openings on the headwall, two exhaust port and two peepholes, all located on top of the flue.
In order to direct only the exhaust gasses into the external fume treatment system and prevent cooler gasses from the upstream end of the adjacent fire group from flowing backwards into the exhaust, a seal is placed inside the flue or headwall. This seal is moved each time the fire group is moved. The design most often used is a flat, flexible, high temperature cloth that is placed across the internal rectangular opening in the headwall. This seal is manually positioned by an operator while on his hands and knees exposed to heat, dust and fumes. As a furnace ages, the effectiveness of this seal decreases as a flat seal cannot be effective on an uneven surface. This leads to higher energy costs due to the need for larger fans to move the increased quantity of exhaust gas. The cooling effect of added ambient air is one of the major factors in decreased efficiency for furnaces of this type.